In parallel with the progress of business activities and the development of interactive learning, projectors have become indispensable image display apparatuses in many fields. The image displaying technology is advancing from the liquid crystal display (LCD) to digital light processing (DLP).
The digital light processing technology employs reflected light, which is modulated by the spatial light modulator (SLM), such as the generally known digital micromirror device (DMD). The DMD is a bistable spatial light modulator consisting of multiple arrays of many pixels. Each pixel has a micromirror. FIG. 1 illustrates a single pixel of DMD. By individually controlling the tilt angle of each micromirror 10, every micromirror 10 can be selectively arranged in two states, namely an “on” state (+θ degrees) 10a and an “off” state (−θ degrees) 10b. Light source is denoted by reference numeral 14. Light reflected by micromirrors 10a in the “on” state passes through the projection lens 16 onto a screen. Light reflected by micromirrors 10 in the “off” state 10b reaches an integrator 18 and a dark field is created. Images are created by gray-scale modulation between the “on” 10a and “off” 10b states. The flat state 10c occurs when the micromirrors 10 are not landed (no deflection). The flat state 10c is not a stable state (not tristable).
Contrast, brightness, resolution, weight, light source lifetime are the indices for determining the quality of a projector. Scattering and diffraction are the dominant mechanisms that determine the contrast. Contrast is defined as the ratio of the light intensities between “on” 10a and “off” 10b states of the spatial light modulator device. Contrast can be enhanced by either increasing the light intensity of the on-state 10a, or by decreasing the light intensity of the off-state 10b. As the brightness of the off-state approaches zero, a small absolute drop in off-state brightness results in a higher impact to the overall contrast. However, owing to scattering and diffraction from the flat state 10c, the brightness of the off-state doesn't actually approach zero. So far, the conventional techniques do not have relatively satisfactory solutions. Thus, the contrast-improving techniques described in this application concentrate on enhancing the contrast by lowering the brightness of the off-state.